1. Field of the Invention
The present invention relates to an image forming apparatus for forming images in an electrophotographic process.
2. Description of the Related Art
In an electrophotographic image forming apparatus, high-quality images can be formed in a short time through simple operations, and moreover a maintenance of the apparatus is easy. The electrophotographic image forming apparatus has been therefore widely used for a copier, a printer, or a facsimile apparatus, for example. The image forming apparatus includes, for example, a photoreceptor, a charging device, an exposing device, a developing device, a transferring device, a fixing device, and a cleaning device. The photoreceptor is a roller-like member which has a surface provided with a photosensitive layer. The charging device is impressed with voltage to charge the surface of the photoreceptor with predetermined potential. The exposing device forms an electrostatic latent image on the charged surface of the photoreceptor by emitting thereto signal light which corresponds to image information. The developing device develops a toner image by supplying a toner to the electrostatic latent image on the surface of the photoreceptor. The transferring device transfers onto a recording medium the toner image formed on the surface of the photoreceptor drum. The fixing device fixes the toner image to the recording medium. As a result, an image is formed on the recording medium. The cleaning device is a blade-like member which is disposed in contact with the surface of the photoreceptor, being used to remove the toner which remains on the surface of the photoreceptor after the toner image has been transferred therefrom.
The developing device used herein includes a developing roller, a developer tank, and a toner concentration sensor. The developing roller supplies the toner to the electrostatic latent image on the surface of the photoreceptor to form the toner image thereon. The developer tank stores therein two-component developer containing the toner to supply the two-component developer to the developing roller. The toner concentration sensor detects the concentration of the toner in the developer tank. In accordance with a result detected by the toner concentration sensor, toner supply into the developer tank is controlled. The toner concentration sensor normally outputs a detection result in the form of voltage. The output voltage of the toner concentration sensor is, however, susceptible to detection sensitivity of the toner concentration sensor itself, an environment (a temperature, humidity, and the cumulative number of prints) in which the two-component developer is used, and the like parameter. For example, the toner concentration sensor exhibits different detection sensitivities depending on a temperature, humidity, or the like parameter. The detection sensitivity of the toner concentration sensor changes depending also on an image printing speed, the number of printed images, etc. in the image forming apparatus. Further, in a color image forming apparatus, the toner concentration sensor outputs different detection results depending even on a toner color. This may cause a failure to supply an appropriate amount of toner to the developer tank, in consequence whereof a lower-density image, a partially scraped image, or the like image may be formed.
Furthermore, in an image forming apparatus having a plurality of process speeds (print speeds), it is necessary to modify an inputted set value in order to adjust the detection result of the toner concentration sensor for the respective process speeds.
For example, in an image forming apparatus disclosed in Japanese Unexamined Patent Publication JP-A 2002-169369, the concentration of the toner in the developer tank is detected upon changing an image resolution and a process speed, and according to the detected toner concentration and the process speed, a threshold value for voltage outputted by a toner concentration sensor is modified.
Further, in an image forming apparatus disclosed in Japanese Unexamined Patent Publication JP-A 2004-361511, when an image forming process speed is modified by a speed-modifying section, a condition-setting section sets a reference value as a set value of image forming condition in an image forming mode serving as a reference while setting a corrected value that is obtained by adjusting the reference value with use of a correction coefficient, as a set value of image forming condition in another image forming mode. After the image forming process speed has been modified by the speed-modifying section, the correction coefficient is revised by a revising section. The correction coefficient thus revised by the revising section is stored in a memory section to be used later on.
According to the related art, a set value of the toner concentration sensor for each process speed is measured in an initial stage before an image formation is carried out, whereby a coefficient relative to a reference process speed is determined for a process speed other than a reference process speed. After the start of the image formation, the toner concentration is measured only for the reference process speed to modify the set value while, for the process speed other than the reference process speed, the coefficient determined in the initial stage is merely used to modify the set value.
Since the coefficient determined in the initial stage is used even after the start of the image formation, a temporal change is not sufficiently taken into consideration, which may result in a failure to appropriately set the set value for another process speed. The failure to appropriately set the set value of the toner concentration sensor leads to a failure to accurately detect the concentration of the toner in the developer tank, which failure affects the toner supply, image density, etc. to cause a decrease in quality of images to be outputted.